The Blue Desert Conundrum

A juvenile short-finned pilot whale peers up from between two adults. The deep blue water in which they swim appears to be empty of any food that could sustain them, but when they surface from a dive they sometimes drop or regurgitate portions of cephalopods that they have captured well below the reach of our vision. Photo by Doug Perrine

A couple of decades ago I pulled up my roots from South Florida and gave up easy access to the beautiful shallow reefs of the Caribbean and Bahamas to move to Hawai‘i. One of the attractions was the proximity of deep, clear, blue oceanic water close to shore, allowing access to photograph some of the little-known animals that inhabit this zone.

Crystal-clear, nutrient-rich waters may exist in polar seas in the darkness of winter, but elsewhere nutrients in the presence of sunlight nourish phytoplankton, which turns the water green with chlorophyll. Phytoplankton feed zooplankton, which transfer the sun’s energy up the food chain. Clear water is nutrient-poor. Corals are able to thrive in clear water because of internal nutrient cycling between their own tissues and their symbiotic photosynthetic algae. But corals require a hard bottom and relatively shallow water to grow.

Deep, blue offshore waters have no coral reefs and little in the way of nutrients, plankton or larger edible items. Apart from the occasional algal bloom in surface waters and the nightly migration of mesopelagic creatures to the surface that fuels the blackwater diving craze, these are marine deserts — vast stretches of ocean that are nearly sterile. And yet this is where I have had encounters with some of the largest creatures on the planet. How can such sterile waters support them?

A juvenile sperm whale comes to the surface. Sperm whales, like pilot whales, dive deep to feast on cephalopods but dive even deeper in search of larger prey.

Where we encounter marine megafauna, we see only a tiny slice of their habitats and lives, which rarely includes feeding. These animals may travel thousands of feet vertically or migrate a few thousand miles horizontally to meet their nutritional needs. Some of them — sperm whales, for example — must do both: descend to depths of up to a mile or more to feast on aggregations of squid and roam across large swaths of the Pacific to avoid depleting their food resources in any one area.

Humpback whales feed on small schooling fishes and crustaceans in the upper water column. Their feeding activities commonly occur in areas where the surface water is cold and nutrient-rich. Most humpback whales that winter in Hawai‘i during their reproductive season spend their summers gorging themselves in Alaska. Sperm whales also require dense prey concentrations to feed efficiently but generally find these by diving to deeper ocean layers, where squid are abundant. We can see sperm whales around the islands any time of year. In winter you might encounter both humpbacks and sperm whales around Hawai‘i, but only the sperm whales are feeding, so they are not competing.

Other species of megafauna reside in Hawai‘i year-round. They may occasionally feed near the surface when cool upwellings bring up nutrients, but most get the bulk of their food at depth. When I moved to Hawai‘i, scientists knew little about the habits of the odontocetes (dolphins and toothed whales) here, so I couldn’t guess how what looks like a monotone environment supported such variety. But two decades of research by the Cascadia Research Collective, led by Robin Baird, Ph.D., have enhanced our understanding. Utilizing a strategy known as niche partitioning, different predators specialize in capturing different types of prey or exploiting different subhabitats, reducing the overlap between their food sources. Other researchers have revealed more secrets of several individual species.

Spinner dolphins are iconic in Hawai‘i, often appearing in shallow coastal bays in the daytime but rarely to feed. Spinners use these habitats, protected from deadly vertical ambush attacks by sharks, to rest and socialize. In the late afternoon they head to deeper waters, where they feed primarily on lanternfish and sometimes squid and shrimp. Lanternfish reside in the mesopelagic boundary layer between 1,300 and 2,200 feet but migrate upward at night to touch the surface, where they are available to dolphins and blackwater divers. Research by Kelly Benoit-Bird, Ph.D., determined that spinner dolphins must consume an average of three to four prey items per minute for 11 hours to meet their caloric needs. Spinners pair up to encircle lanternfish schools and take turns picking them off in a coordinated feeding ritual that sonar imaging shows as resembling a square dance.

Hawaiian spinner dolphins rest in shallow water by day but go offshore at night to feed in deep water.

Data from Cascadia’s depth-transmitting satellite tags indicate that rough-toothed dolphins also eat fish and squid in the mesopelagic boundary layer at dusk and night. They also actively feed during the day, however, exploiting small surface-dwelling fishes of oceanic waters such as flying fish and needlefish. These dolphins sometimes catch fish as large as dorado or mahi-mahi. Clever and adaptable, they have learned to pilfer morsels of fish from other predators, including false killer whales and human fishers, and have learned to take advantage of abandoned nets and fish aggregation devices (FADs). While usually in the upper 100 feet of the water column during the day, they may make deep dives to feed.

A dolphin that can reach nearly 20 feet long, false killer whales also feed on epipelagic (open-ocean surface waters) fish. While they share with rough-toothed dolphins some prey such as mahi-mahi, there is little overlap because they can take much larger fish, including wahoo, swordfish and tuna weighing more than 100 pounds. False killer whales utilize a unique hunting strategy. The pod breaks into smaller groups that spread out to look for prey. When an individual makes a kill, it announces its catch with noisy breaches, and other pod members converge to share the meal. False killer whales in Hawai‘i will sometimes stun their prey by batting the fish into the air with their flukes.

Short-finned pilot whales, often confused with false killer whales, are another type of “blackfish” — the six dolphin species commonly called whales. Pilot whales have different body shapes and behavior. They generally move slowly and rest when at the surface; their diet consists mainly of deep-water squid. They dive below 1,000 feet to feed on squid, octopus and fish, but their dives get progressively shallower as night brings mesopelagic prey closer to the surface.

Oceanic whitetip sharks sometimes follow pilot whale pods. We don’t fully understand the relationship, but the sharks probably opportunistically capitalize on the whales’ prey and have been filmed consuming whale regurgitations and feces. The sharks have an extremely broad diet, take advantage of any nutrition available and minimize the energy they expend to hunt. In Hawai‘i they are often near FADs to prey on injured fish, scavenge discarded bait and released catches, and prey on other fish the FADs attract. They inhabit open-ocean surface waters down to 500 feet or more.

Oceanic whitetip sharks use their oversize pectoral fins to glide through the water, conserving energy while searching for meals that are few and far between.

Galápagos sharks also have a broad diet but roam closer to shore and hunt near shallow and deep reefs as well as surface waters. The stomachs of Galápagos sharks that researchers have caught in Hawai‘i contained a variety of bony fish, cephalopods, sharks, rays, crustaceans and mammals.

We mostly see scalloped hammerhead sharks, like humpback whales, during their reproductive season, when they come into shallow waters to mate and pup. Along the Kona Coast, schools of female hammerheads sometimes congregate in near-shore waters over flat, sandy bottoms at depths of about 200 feet. These areas are devoid of apparent prey items. Tagging studies show that when the hammerheads leave, they move over the drop-off down the reef slope and follow the steep contour up or down the coast, presumably feeding on bottom-dwelling fish and invertebrates, as this species does in other parts of the world. Hammerheads caught in Hawai‘i have had beaks of pelagic squid in their stomachs, indicating the ability to hunt offshore. A few adults were found to have eaten pups of their own species.

The Kona Coast is also noted for the number and size of blue marlin caught in its waters. These fish prefer warm, offshore surface waters above the thermocline, which are generally clear and nonproductive. The explanation for this seeming contradiction is cyclonic eddies. As strong currents pass by the northern and southern tips of Hawai‘i Island, they are refracted inward and propagate eddies that can spin in either direction. A cyclonic or counterclockwise eddy, also known as a cold-core eddy, draws up cool, nutrient-loaded water in its center, kicking off an explosion of life and attracting the small tuna and other fish that become prey for marlin. A clockwise or anticyclonic eddy has a warm core where surface water is pushed downward and creates poor fishing conditions. Like many other marine megafauna species, marlin cross the blue offshore desert in search of the scattered oases that nourish life.

Bottom currents that are forced upward when they strike shallower ridges and pinnacles surrounding the islands can also produce nutrient upwellings. These upwellings can cause linear surface slicks sporting long brown patches of algae and abundant microfauna. Following these slicks can sometimes result in encounters with megafauna such as manta rays.

With space-based environmental monitoring and improved technology, natural history photographers of the future may be able to find aggregations of wild marine animals and deploy remotely operated camera platforms to film them at depth, at high speed and without disturbing their natural behavior. For now, I am stuck with 20th-century technology. I spend long, mostly fruitless hours cruising a seemingly endless expanse of blue underwater desert, hoping to stumble upon something of interest. I accumulated the photos accompanying this article over a quarter-century of occasional encounters that, despite their rarity, made it all worthwhile.

Explore More

See more of Doug Perrine’s megafauna images in this bonus photo gallery.

© Alert Diver — Q3/Q4 2021